mmc: sdhci-s3c: fix the wrong register value, when clock is disabled

When use the QUIRK_NONSTANDARD_CLOCK, then never set to 0 at clock control
register.  This patch fixes this problem.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>